This invention relates to a projection exposure apparatus and a device manufacturing method, for photolithographically transferring a fine pattern formed on an original onto a substrate such as a wafer, for example.
The fine pattern forming procedure for a semiconductor device or a liquid crystal display, for example, uses a projection transfer technology, called photolithography. The projection transfer is performed in the following manner. An original pattern formed on a quartz glass substrate, called a reticle or a mask, is illuminated. In response, through a projection optical system, a latent image pattern is photolithographically transferred onto a substrate such as a semiconductor wafer or a liquid crystal forming glass substrate, for example. The latent image pattern is then developed into a resist pattern. Thereafter, an etching process of a high processing selection ratio between the pattern and the base material surface underlying the resist is performed, whereby the substrate is microprocessed.
In the projection exposure process included in such a fine pattern forming procedure, particularly for production of a semiconductor device as represented by current MPU or DRAM wherein extraordinary minuteness and very high processing precision are required, a reduction projection exposure apparatus called a stepper is mainly used. The stepper is a step-and-repeat type exposure apparatus wherein equivalently divided exposure regions (exposure shots) on a wafer are sequentially moved into an exposure picture angle below a projection optical system by means of a wafer carrying stage, whereby pattern exposures are repeatedly performed.
There is a step-and-scan type exposure apparatus, called a scanner. In this type of exposure apparatus, a wafer and a reticle are scanned and exposed while being scanningly moved relative to a projection optical system having a rectangular illumination region. As compared with a stepper, it has a wider exposure picture angle, and the pattern uniformness is higher.
In any of the stepper and the scanner, in order to meet requirements, of miniaturization of a semiconductor, improvements of the resolving power of a projection optical system have been desired. Many attempts have therefore been made in the development and products.
Examples of conventional measures for improving the resolution of a projection optical system are enlarging the numerical aperture (NA) of a projection optical system while holding the wavelength fixed, and shortening the exposure wavelength such as from g-line to i-line or to the emission wavelength of a KrF or ArF excimer laser, for example. Also, there is a shape changing illumination method in which the shape of an illumination light source is changed to enhance oblique incidence illumination light, or a phase shift mask method wherein a phase difference is produced in transmission light between adjacent reticle patterns. These are attempts to extend the process limit in the optical exposure.
With the improvements in the resolving power, the semiconductor process requires a more strict control precision, while the process margin such as, for example, the depth of focus of a projection optical system and a total overlay tolerance, is being reduced. On the other hand, separately from the improvements of resolution, improvements of the overlay precision itself have been required. This is for the reason that improving the overlay precision leads to narrowing the layout margin which enables reduction of the device size. This leads to an increase of device yield rate per a unit substrate, and thus to a decrease of the cost.
In projection exposure apparatuses, in order to meet these requirements, improvements are being made in regard to an exposure focus system and an alignment system which is directly influential to the overlay precision. Now, a conventional example of an exposure focus system will be described and, after that, an alignment system influential to the overlay precision will be described.
A focus detecting unit in am exposure focus system generally comprises an off-axis type wherein a probe light is obliquely projected onto a surface to be detected and wherein the focus detection is made on the basis of the position where the reflected light is collected. Usually, the detecting unit is fixed at the peripheral portion of an image plane of the projection optical system. In a stepper, after an exposure shot is positioned within the exposure picture angle, a wafer stage is moved upwardly/downwardly and tilted (for focusing) on the basis of the tilt amount and the level of the wafer surface as measured by the detecting unit, whereby the imaging plans of the projection lens and the plane of the transfer region are brought into registration. The exposure is then performed. In a scanner, the measurement and the focusing process described above are performed simultaneously with the scan exposure.
The wafer alignment is definitely a dominant factor of the overlay precision. The alignment system comprises an alignment detecting unit for measuring alignment marks formed on a wafer, and aligning means for positioning each shot at the exposure position on the basis of the results of processing the measured values of the positions in accordance with a predetermined method. The former alignment detecting unit measures the position of an exposure shot on the basis of the positions of alignment marks formed adjacent to that exposure shot. As regards the detection method, there is a TTL method wherein the position measurement is made through a projection optical system, and an off-axis method wherein the measurement is made without intervention of a projection optical system. In both of these methods, for the detection, a focusing operation for bringing the wafer alignment mark into registration with the detection plane is necessary. The measurement of the detection plane level is made also by using the focus detecting unit of the projection optical system or, alternatively, the alignment system itself has a focus detecting device.
As regards the aligning means, there is a die-by-die method wherein, for every exposure shot, the exposure position is measured and the alignment operation is made. However, at present, an alignment method called a global alignment method is used in many cases. In this alignment method, position measurement is made to sample shots of an appropriate number designated beforehand, and all the shot positions are estimated by preparing a linear correction formula to the positions on the basis of the results of measurements. By the position correction formula based on the global alignment method, not only the wafer shift component but also the magnification, the orthogonality and the rotation of the wafer as a whole related to the shot layout can be corrected. Further, depending on the measurement point, the magnification and the rotation of the shot itself can be corrected. As described, the global alignment method has superior advantages such as higher throughput and precise alignment, for example. Additionally, because the alignment operation is made to the whole wafer surface region in accordance with the same correction formula, the state of alignment can be detected once measurements are made to a few points on the substrate. Thus, this method is superior also in respect to the easiness in use.
On the other hand, when the positional deviation between exposure shots has no linearity to the position, namely, when it has a non-linearity, the non-linear deviation swerving from the linear correction amount directly leads to an alignment error, which causes degradation of the overlay precision. Further, when a non-linear deviation is produced at a sample shot position, it causes an error in the linear correction formula to be determined on the basis of the sample shot position. Therefore, with respect to the improvement of the overlay precision, reducing the non-linear deviation between exposure shots is an important matter.
As described above, the overlay precision is becoming more strict and the necessity of reducing the aforementioned non-linear deviation is becoming larger. Thus, the subject range for factors which are attributable to the production of non-linear deviation, to be suppressed, becomes wider and wider. Particularly, one of the factors which recently became obvious is the distortion along the wafer surface direction caused at the same time of wafer chucking. This is phenomenon that local extension/contraction is produced in the wafer surface to cause shift of the position of an alignment mark or a pattern to be printed.
In a projection exposure apparatus, generally, a wafer is vacuum attracted to a pin contact chuck having an assured flatness, to thereby perform flatness correction of the wafer. However, if a foreign substance is trapped between the pin and the wafer or when the surface irregularity shape of the wafer contact face is changed by presence of any deposition, resulting from a film forming process such as a CVD process, for example, or, alternatively, when the state of contact between the pin and the wafer surface irregularity shape is changed due to a change in positional relationship between the chuck and the wafer through repetition of the exposure process, distortion may be caused in the wafer due to a bending moment produced by the attraction reactive force of the wafer contact face. On such an occasion, in accordance with the distortion distribution produced in the wafer, there occurs a non-linear shift between exposure shots, causing degradation of the overlay precision.
The cause for such non-linear shift will be explained with reference to FIG. 6.
FIG. 6 shows a state in which distortion is produced in a wafer placed on a pin contact chuck, due to a foreign substance trapped therebetween. Denoted at P1, P2, . . . , are pins of the pin contact chuck. Denoted at W is a wafer of a thickness 2h, and denoted at D in a foreign substance which is trapped between the pin P2 and the wafer W. The reference plane for the pin height is defined by a plane O-R, and the detection plane to be considered here is a plane B-B1. If there is no trapping of a substance D, the wafer W will be held by the pins P1, P2, . . . , so that the plane O-O1 on the pin P1, the detection plane B-B1, the plane A-A1 on the pin P2 will be parallel to each other. However, if there is a foreign substance D trapped upon the pin P2, because of the presence of this substance D, a bending moment is produced in accordance with the difference in level between the supporting points O and A for the wafer W. As a result of it, the detection plane B-B1 is tilted by a small angle xcex94xcex8 with respect to the plane O-O1. Here, the plane O2-B2-A2 depicted by a dash-and-dot line in the drawing, at the center of the section of the wafer W, is a neutral plane where no extension/contraction is produced by the bending moment. Here, (1) if, from the second order or higher of the angle xcex94xcex8, a change in distance along the pin height reference plane direction due to the bending moment is disregarded and (2) if it is assumed that the plane B-B1 is orthogonal to the neutral plane O2-B2-A2 (Bernoulli-Navier hypothesis), than the shift xcex94r to be produced by the trapping of the foreign substance D can be expressed by the following equation:
xcex94r=hxxcex94xcex8
The small angle xcex94xcex8 represents the tilt of the wafer surface with respect to the pin height reference plane O-R, that is, the chuck reference plane. Therefore, if the small angle xcex94xcex8 can be detected at the alignment mark surface, the aforementioned non-linear shift amount may be calculated and it say be subtracted from a measured value, by which a proper shot position linear correction formula can be obtained. Thus, the alignment precision can be improved.
However, alignment systems in conventional exposure apparatuses involve the following problems.
(1) Conventional exposure apparatuses are not equipped with a function for detecting a small angle xcex94xcex8 at a surface adjacent to an alignment mark.
(2) Although a small angle xcex94xcex8 at a surface adjacent to an alignment mark could be, for the present, measured by using a focus system for detecting the surface to be exposed, if the angle xcex94xcex8 is measured before or after the measurement through an alignment detection system, the wafer position becomes different for the alignment measurement and for the focus measurement. This necessitates the addition of time for moving the wafer to the focus measurement position and, as a result of it, the throughput of the exposure apparatus is lowered.
Further, since in the focus system for detecting the surface to be exposed, the distance between the wafer surface level measurement points is set to be approximately equal to the exposure shot region length (10-20 mm), in order to measure a tilt of each local alignment mark region of a size of about 0.1 mm square, the measurement process has to be done plural times while changing the wafer position to the same extent. In this case, the focus measurement time increases considerably, which results is degradation of the throughput.
(3) There is a more serious problem. It is a non-linear positional deviation between the time for forming an alignment mark by exposure and the time for measuring the alignment. More specifically, the small angle xcex94xcex8 at a surface adjacent to an alignment mark, which is the subject of measurement for correction of the shift amount, corresponds to the difference between a tilt xcex94xcex8pr of the alignment mark surface when it is formed and a tilt of xcex94xcex8po at the time of measurement.
Therefore, it is necessary to measure the tilt xcex94xcex8pr at the time of alignment mark formation, to store the value xcex94xcex8, as a hysteresis value, to read out the hysteresis value xcex94xcex8pr upon measurement of the alignment mark position, and to calculate the value of a small angle xcex94xcex8 from the values xcex94xcex8pr and xcex94xcex8po. However, measurement of xcex94xcex8pr involves similar problems as has been described in Items (1) and (2) above. Further, conventional exposure apparatuses are not equipped with a function for storing and reading out xcex94xcex8pr.
As regards a non-linear shift of an exposure shot position itself, like the alignment it can be corrected on the basis of s small angle xcex94xcex8a of the shot surface. Practically, in the exposure process, detection and correction of the small angle xcex94xcex8a is performed by using an exposure focus system. However, conventional exposure apparatuses are not equipped with a function for correcting the exposure shot position on the basis of the small angle xcex94xcex8a. Further, like the matter described in item (3) above, what is the problem to be considered in respect to the overlay precision is, more exactly, a non-linear shift to be produced between a preceding exposure for forming a pattern on an underlying layer in a particular exposure shot and a current exposure of a resist lying thereon in that shot. More specifically, the small angle xcex94xcex8a of the shot surface which is the subject of measurement to be done is defined by the difference between a tilt xcex94xcex8arp of the surface to be exposed, at the time of a preceding exposure for forming a pattern on an underlying layer, and a tilt xcex94xcex8apo of the surface to be exposed, at the time of a current exposure of a resist lying thereon. On the other hand, while the tilt xcex94xcex8apr of the surface to be exposed is detected by a focus system at the time of exposure for forming a pattern on an underlying layer, it is not stored as an exposure hysteresis. Therefore, as accurate tilt xcex94xcex8a can not be detected by calculating the difference between xcex94xcex8apr and xcex94xcex8apo.
It is accordingly an object of the present invention to provide a projection exposure apparatus and/or a device manufacturing method by which a non-linear shift of an alignment mark of an exposure shot due to distortion of a substrate such as a wafer, along a substrate surface direction, to be produced when the substrate is chucked, can be calculated accurately to enable position correction and thus to improve the overlay precision.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.